One common method for printing images on a receiver member is referred to as electrography. In a particular implementation of this method, known as electrophotography, an electrostatic image is formed on a dielectric member by uniformly charging the dielectric member and then discharging selected areas of the uniform charge to yield an image-wise electrostatic charge pattern. Such discharge is typically accomplished by exposing the uniformly charged dielectric member to actinic radiation provided by selectively activating particular light sources in an LED array or a laser device directed at the dielectric member. After the image-wise charge pattern is formed, the pigmented (or in some instances, non-pigmented) marking particles are given a charge, substantially opposite the charge pattern on the dielectric member and brought into the vicinity of the dielectric member so as to be attracted to the image-wise charge pattern to develop such pattern into a visible image.
Thereafter, a suitable receiver member, sometimes simply referred to as a receiver, (e.g., a cut sheet of plain bond paper) or an intermediate receiver member, sometimes simply referred to as an intermediate, (e.g. a compliant or non-compliant roller or web) is brought into juxtaposition with the marking particle developed image-wise charge pattern on the dielectric member. A suitable electric field is applied to transfer the marking particles to the receiver member in the image-wise pattern to form the desired print image on the receiver or intermediate receiver member. In the case of an intermediate receiver member, a secondary transfer step is performed whereby a second suitable electric field is applied to transfer the marking particles from the intermediate receiver member to the receiver member. The receiver member is then removed from its operative association with the dielectric member and the marking particle print image is permanently fixed to the receiver member typically using heat, pressure or and pressure. Multiple layers or marking materials can be overlaid on one receiver, for example layers of different color particles can be overlaid on one receiver member to form a multi-color print image on the receiver member after fixing.
The use of toner particles, also referred to as marking particles, in electrophotographic printing, to create a raised surface or other specialized image, in some cases, has led to poor quality prints, machine contamination issues, and color shifts. For instance, the addition of a clear toner in these regions to provide a raised print having tactile feel increases the total mass per unit area of toner that needs to be fixed to the receiver member to levels greater than in the past. For a roller fusing system this necessitates high fuser roller surface temperatures and long fuser nip dwell times to achieve good toner adhesion for the high toner mass laydown regions, especially when the receiver member is a heavyweight (such as a weight of greater than 180 gsm) uncoated paper. Unfortunately, this results in substantial hot offset artifacts in the lower toner mass laydown regions, e.g. non-raised areas, creating ghost images in multiple sheet printing jobs and thus reducing the fuser offset latitude. The fuser offset latitude is the range of temperatures between the lowest temperature where the toner will stick to the receiver at maximum laydown and the highest temperature where the toner sticks to the receiver and does not stick to the fuser roller at low and intermediate laydowns. The hot offset also greatly increases the contamination of other rollers associated with the fusing subsystem such as the donor and metering rollers used to apply a release agent such as silicone oil to the surface of the fuser roller, greatly increasing the maintenance requirement of these rollers so as to prevent image artifacts. Furthermore, during the fusing process the high laydown of clear toner inhibits the flowing and coalescing of the toner layers underneath, allowing the receiver member to appear through the gaps in the discrete toner particles. This reduces the level of color saturation, creating an unwanted shift in color when comparing the same image area, raised versus non-raised.
A related problem may be encountered when trying to fuse layers of toner onto a dense or coated receiver member, particularly members that do not readily absorb the oil often used as a release agent in roller fusing systems. Often the fuser temperature and nipwidth must be greatly increased so as to provide adequate adhesion of the toner layers onto this type of receiver. These extreme fusing conditions may result in hot offset of the toner onto the fuser roller, again causing the problems described above, often resulting in very little or no fuser hot offset latitude.
In order to improve image quality and reduce maintenance of the fuser subsystem, as well as increase the range of fusable receiver members, a method for increasing the fuser offset latitude is needed.